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Current density and resistance in piezoelectric device

February 4, 2013, 1:42 am
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I am trying to simulate the current density in a piezoelectric device.
The geometry is a simple rod with a fixed boundary (also ground boundary) and a surface load on the opposite boundary. I can easily get the displacement and voltage, due to the applied force.

If I plot the current density (normal or in any direction) the result is simply zero... due to the polarisation of the structure (due to the applied force) the current cannot be zero?

Secondly, I want to add resistance to the material properties, how can I do this? It is not a required parameter to solve the equations, I guess I have to alter the equations and add the resistance on that level?
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Surface growth and moving mesh (ale) ?

February 4, 2013, 3:36 am
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Hi,

I am building a CVD growth model using the heavy species transport, much similar to the silane growth model. My geometry includes a small trench (um scale), where I deposit the material. Is it possible to use the moving mesh system to actually move the edges of the trench according to the growth rate? In this model the setting does not seem to be as straightforward as in electrodeposition models, so if it is possible then how would this be done?

Thanks,
Lauri Lehtola
Aalto University
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"Error: Duplicate variable name."

February 4, 2013, 4:40 am
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I am using solid mechanics, electrostatics and moving mesh and I want to perform an Eigenfrequency Study. The solver crashes with an error message:

"Error: Duplicate variable name."

But I didn't find any variable names to duplicate.
Below is my file.
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stability of geometry creation

February 4, 2013, 6:03 am
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Suppose that the geometry is not simple enough to be represented by regular geometry primitives.
Then, there are two chooses:
1- use of parametric surfaces (possible as external function)
2- use of import geomerty (i.e. STL file which is externally created)

use of parametric surfaces are in my case relatively slow. So I concentrated on STL import.
I externally create STL file (a surface defined by triangles). I can convert it to a solid in comsol. But I cannot use this solid in difference or union operation later on in a stable manner. I almost always get a domain decomposition error.

In general, even for standard models, one needs to play with the relative repair tolerances to get to desired final geometry (which is not practical it is an automated model). If the geometry creation fail, all automated model fails.

Did anyone of you had similar problems? Any solution suggestions?
Does comsol have any plan to improve the stability of geometry creation?

regards,
dr.ing.- yalcin kaymak
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Calculating contact area

February 4, 2013, 1:24 pm
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Is there any way to calculate the contact area of a contact pair within COMSOL (ie without having to post-process data in MATLAB, which I have yet to try)? I have tried surface integration and other options under "Derived Values" to no avail.
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Unphysical solution with ellipsoid/sphere vs. hemisphere

February 4, 2013, 8:17 pm
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Having a problem with COMSOL providing unphysical solutions when I use a sphere or ellipsoid "hovering" above the bottom of a domain as opposed to a hemisphere.

I have a box, and within the box is a source (flux boundary condition) and a sink (zero concentration boundary condition). The model provides realistic and physical solutions to concentration in the box and the total flux across the source and sink when they are modeled as hemispheres out of the bottom of the box. However, if I create spheres or ellipsoids which are protruding partially out of the bottom of the box (so that the angle between the sphere/ellipsoid and bottom is acute) the solution is no longer physical. The system equilibriates with the total flux of the sink far below that of the source, which is not possible. Bringing the sphere/ellipsoid up off the bottom completely, improves the error, but does not eliminate it. Also, ellipsoids are consistently worse than spheres. Depending on whether I make the source or sink an ellipsoid the flux either way undershoots or way overshoots and only appears accurately when I use the hemispheres. I don't even understand how the model can reach equilibrium (with constant concentration at a medium distance point) with uneven fluxes.

I know this sounds really general but I am getting very consistent errors, by trying these variations out, all other things being the same. I would like some suggestions on how to improve the accuracy here and eliminate this problem as ideally I would like to model an ellipsoid source with acute angle to box, and spherical sink.

Thank-you for your time.

I have attached two png files of not so bad solutions comparing hemispheres (good converging fluxes) and spheres just above the surface (moderately bad solution). It gets much much worse with acute angles and ellipsoids.

EDIT: Added an ellipsoid solution.. you can see the fluxes stabilize at wildly different values, yet the concentration in the box stabilizes.. this is obviously not physically possible with more molecules coming in than going out.

EDIT2: Realized that I was using total flux magnitude instead of normal total flux.. I am not clear on what exactly total flux magnitude is then. The results are 'more' realistic now but the fluxes still do not sum to zero
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S-Parameter in Complex format in COMSOL

February 4, 2013, 9:23 pm
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I have not yet been able to find how COMSOL represent complex format of S-parameters. Can anybody please provide the correct format with reference?

I am wondering if it is like S11=Re(S11)+j*Im(S11) or S11=Re(S11)-j*Im(S11)
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How to incorporate varying temperature with respect to time??

February 4, 2013, 9:26 pm
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As the ambient temperature varies with time, I wish to add that variation pattern of temperature in COMSOL 4.3a. Is it possible? If possible, how can I add

1. using equations
2. using excel sheet for that particular entry alone.
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Root mean square in 4.3a

February 5, 2013, 3:08 am
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Hi

I need to calculate the Root mean square (RMS) of a voltage. Are there any predefined functions in Comsol to do so. I searched the docs and got no clue on that.

Thanks a lot
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Licensed number of users already reached

February 5, 2013, 4:13 am
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Hello,

I couldn't open the versions 4.2a and 4.3 from COMSOL. A message informing me that the number of users already reached and the program closed automatically. It seems strange for me, because anyone was running it in that moment. I've purchased the licenses from both versions last year. I looked for an answer in COMSOL web site, but unsuccessfully. They said that my subscription has already expired since September 1, 2012.

Does anyone know how to solve this problem?

Thank you very much.

Best regards.
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2D electrostatic Force

February 5, 2013, 4:32 am
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Hi,

I am modeling a slice of a simple comb capacitor in 2D. I would like to determine the force acting on the fingers. I can calculate the electric field, but when I try to determine the force through Fy=dE/dy along the line it always returns zero. Please find the attached model.
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Is the problem positive definite? Floquet periodic boundary conditions or PML using

February 5, 2013, 5:30 am
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Hello, I am doing some elastic wave simulations using solid mechanics module and piezoelectric module. And sometimes the DOF of my models are quite large so that I have to choose the solver manually to save memory and time. I have read the Comsol Multiphysics Reference Guide and realized that the choice of the best solver is depend on the type of the equation.
Here is my situation: I use Floquet-type periodic BC in eigenfrequency study and PML in frequency domain analysis very often, and because my lack of mathematical knowledge I don't know whether the problem is positive definite or not under these kind of boundary settings. (They are not ordinary boundaries that appear in my math textbooks) So could anyone please help me? Thanks!
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How to rotate a 2D Plot?

February 5, 2013, 7:16 am
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I built a model with a 3D geometry. Unfortunately the created 2D plot (postprocessing), which is based on a cut plane, is rotated by 90°. Is it possible to rotate the 2D plot back by 90° or to exchange the axis (x/y -> y/x)?

Thank you in advance!
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What physics do I need to model clogging of a vein? (very similar to electrodeposition tutorial)

February 5, 2013, 8:38 am
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If I have a species entering a vein at some velocity, how would I model the reaction with the surface and thereby build up of particles on the wall?

I know I need some mass transfer equation (let's say dilute species module) and I need laminar incompressible Navier stokes module.

What other modules/physics would I use to model the reaction with the wall and subsequent build up of particles
and
would I just use "moving mesh" module similar to electrodeposition module?

A list of suggested physics or examples would be of great help. This set-up should be very similar to the electrodeposition example

Thank you
Alex
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Point Charge

December 21, 2011, 2:34 pm
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Hello all

I want to define point charges (lets say some circles as positive Or negative charges) in Comsol. Can anybody give me some idea how i can do this? what I have in my mind is:

define a constant as Q and equal that to a number (like 1) and then place Q or -Q as the value in space charge density when defining material properties.

However I face in to some problems:

1- I cam only find surface charge density (and not space) in 3d.
2- when i choose Q as surface charge density it seems it doesn't work

anybody have any idea?

Bahar
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Using interpolation to define temperature-dependent material properties

December 30, 2012, 8:53 am
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Hi,

I am a COMSOL beginner. Trying to added a file containing temperatures and measured material properties and then utilize an interpolation function to define the temperature-dependent material properties. I have attached both the COMSOL file and the material property file. The first and third columns contain the temperatures and the 2nd and 4th columns are the corresponding measured properties. (I've attached the 'xlsx' file as I couldn't attached the 'csv' file I used in the model)

Any advice will be appreciated. Perhaps there already exist examples that I can follow to complete my model.

Thanks in advance.

KB
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How do I make a laser on z-axis in 2D modeling?

January 24, 2013, 11:22 pm
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Hi,
I did 2D modeling in Electromagnetic waves, Transient mode.
I wanna make a model in which the enhanced electric field is shown when a laser passes from air to nanopattern.
however i don't know how to make a laser passing through nanopattern z-axis.

actually I am modeling plasmonic effect in gold nano bowtie, you know, and I read 'Second Harmonic Generation of a Gaussian Beam' in model library, but I didn't apply this sample into my model.
in shorts, i cannot change x-axis laser into z-axis laser....

question: 1. How do I make a laser on z-axis in 2D modeling?
2. How do I apply model depth on z-axis in 2D?
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MAKING STRIATIONS IN A SOLID CYLINDER

January 29, 2013, 2:08 pm
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how do i make striations in a solid cylinder? I know it should be possible but some how cannot figure out how?
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How to enter the temperature that varies with time??

February 3, 2013, 9:02 pm
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Due to the variation in air velocity, the external temperature varies with time. Can I know how can I input the values using an equation or an excel sheet??
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voltage at different coordinates

February 5, 2013, 5:13 pm
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Hello every one.

it is possible in COMSOL to fine the voltage at (x,y,z) coordinates?

thank you
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